Field of the Invention
This invention relates to an apparatus and process for recovery of elemental sulfur. More specifically, this invention provides an apparatus and process for recovery of elemental sulfur and removal of sulfur-containing contaminants from an acid gas stream.
Background of the Invention
The sulfur recovery industry has been using the Claus gas phase reactions as the basis for recovering elemental sulfur from hydrogen sulfide (H2S) since the 1940s. The Claus plant, the long-standing ‘workhorse’ of the industry, uses the Claus chemistry to achieve between 96% and 98% recovery of elemental sulfur from the H2S in an acid gas streams. The gas phase elemental sulfur from the Claus plant is subsequently condensed and recovered in the liquid form.
The vast majority of all operating Claus plants worldwide include a thermal stage (i.e. a free-flame reaction furnace and a wasteheat boiler) followed by either 2 or 3 catalytic converters or catalytic stages that result in recovery efficiencies of 96% for a 2-stage design or 98% for a 3-stage design. There are only a handful of 4-stage designs in the world because early on the sulfur recovery industry recognized that a 4th catalytic stage only marginally increased the recovery efficiency above 98% and was therefore uneconomical.
Owing to the negative impact of acid rain, formed due to high levels of sulfur dioxide (SO2) in the atmosphere, emissions controls required limits on the amount of SO2 emitted in the effluent of the Claus plants. In response, the industry began developing Tail Gas Treatment (TGT) technologies to be placed immediately downstream of the Claus plant to further improve the recovery efficiency of the sulfur recovery unit to above 99%, or in some cases above 99.9%, while removing SO2 from the effluent.
By far the most common combination of Claus plant and TGT for achieving greater than 99.9% recovery is a Claus plant followed by a reduction/absorption amine-based technology. This technology requires the reduction and hydrolysis of sulfur bearing compounds back to the form of H2S to allow for absorption in an amine contactor. The H2S that is absorbed into the amine is then regenerated and sent back to the front end of the Claus plant as a recycle acid gas stream.
In addition to recovering elemental sulfur, Claus plants also destroy contaminants present in acid gas streams. Contaminants include C1-C6+ hydrocarbons, benzene, toluene, ethyl benzene, and xylene (BTEX), methanol (CH3OH), and ammonia (NH3). The thermal stage of the Claus plant plays a critical role in the destruction of these contaminants. If these contaminants are not properly destroyed in the thermal stage they will negatively impact the purity of the sulfur product and can cause problems in the downstream units and even possibly end up being emitted to the atmosphere.
While the Claus plant does provide a path for recovery of sulfur, it is not without drawbacks. The catalytic stages require regeneration due to catalyst fouling and plugging. The regeneration can take significant downtime, potentially putting the entire processing unit offline. The catalytic stages are sensitive to the presence of contaminants and to the temperature of the catalytic feed stream. These sensitivities can make controlling the catalytic stages cumbersome.
A process which achieves a greater than 99% recovery of sulfur without the presence of the catalytic stages is desirable. In addition, a process that provides a minimal equipment list, reduced plot plan, and ease of operation, including increased reliability is desired.